Driven pulley with flyweights effective at low speeds

ABSTRACT

In a driven pulley used in a variable speed transmission adapted to receive a trapezoidal belt by which power is transmitted from a driving pulley to the driven pulley having a fixed flange, a movable flange and an internal helicoidal spring, there is provided a set of flyweights forced by torsion springs on a conical ring located at the rear of the movable flange. At the minimum rotation speed, the driven pulley generates a high gripping force but as the rotation speed increases, the centrifugal force acting on the flyweights lowers the force exerted by the trapezoidal belt. At a given high rotation speed, the flyweights leave the conical ring and no longer contribute to counterbalancing the radial force exerted by the trapezoidal belt, the helicoidal spring and the internal resultant forces acting alone. The improved driven pulley generates a high initial gripping force while having a reasonably low gripping force at high speed for good efficiency.

RELATED APPLICATION

The present application is a continuation-in-part of the applicationSer. No. 08/084,284 filed on Jun. 28, 1993, now U.S. Pat. No. 5,358,450.

FIELD OF THE INVENTION

The present invention relates to a driven pulley used in a variablespeed transmission adapted to receive a trapezoidal belt by which poweris transmitted from a driving pulley to the driven pulley.

BACKGROUND OF THE INVENTION

Trapezoidal belt variable speed transmissions are commonly used on smallvehicles such as snowmobiles, scooters or small cars. Such transmissionscomprise a driving pulley, a trapezoidal belt and a driven pulley. Thedriving pulley is linked to an engine and the driven pulley is usuallymechanically connected to ground traction means, such as wheels ortracks.

The main object of using a variable speed transmission is toautomatically change the winding diameter of the trapezoidal belt aroundthe driving and the driven pulleys in order to have a maximum torque atlow speeds and a reasonable engine rotation speed at high speeds. Thesides of the trapezoidal belt are, on each pulley, gripped between twoopposite flanges wherein one is fixed and one is movable. At low speeds,the winding diameter of the driving pulley is small and the windingdiameter of the driven pulley is maximum. As tile rotation speed of thedriving pulley increases, the movable flange of the driving pulley getscloser to the fixed flange and thus forces the trapezoidal belt to windon a greater diameter. Since the length of the trapezoidal belt is notsubstantially stretchable, the trapezoidal belt exerts a radial forcetowards the center on the flanges of the driven pulley in addition tothe tangential driving force. This radial force constrains the drivenpulley to have a smaller winding diameter. Therefore, the movable flangeof the driven pulley moves away from the fixed flange until the returnforce exerted by a spring counterbalances the radial force exerted bythe trapezoidal belt. It should be noted at this point that a change inthe load also produces a variation of the winding diameter of thepulleys, a greater load inducing a greater winding diameter of thedriven pulley.

When the rotation speed of the engine decreases, the winding diameter ofthe driving pulley decreases and the radial force exerted by thetrapezoidal belt decreases, thus allowing the driven pulley to have agreater winding diameter.

An example of such a variable speed transmission is disclosed in U.S.Pat. No. 3,266,330.

One of the drawbacks of conventional variable speed transmissions isthat the driven pulley do not always set the trapezoidal belt at themaximum winding diameter when the vehicle is stopped very rapidly from ahigh speed, especially when an important brake force is applied on thewheels or tracks. The trapezoidal belt will then be stuck somewherebetween the high speed position and the initial position. At restart,the ratio between the pulleys will not be optimum and the transmissionwill not deliver full torque.

The above-mentioned drawback is due to the physical limitations imposedby the use of a spring and cam system in the driven pulley tocounterbalance the radial force exerted by the sides of the trapezoidalbelt on the flanges. The spring is usually a helicoidal spring and ismounted between the movable flange and a fixed part. The cam is usuallymounted around the spring and converts the torque exerted on the movableflange to axial gripping force on the belt.

In an ideal driven pulley, the gripping force on the belt at start ismaximum because the belt pull is high to transmit engine torque on asmall driver radius. At higher speed, the belt pull is lower and thegripping force must be reduced to maintain good efficiency.

In a conventional driven pulley, the spring and cam system provide agripping force that reduces as speed increases, as long as the enginesupplies power through the transmission. However, during braking, theengine absorbs power through the transmission. This unloads the cam andthe gripping force is then supplied mainly by the spring. In thissituation, the spring exerts less and less gripping force on the movableflange as it moves closer towards the fixed flange. This low grippingforce is usually insufficient to force the belt towards a higher windingdiameter during a rapid braking action. To generate enough grippingforce, it would require a stronger spring. However, the gripping forcewould then be too high at high speed, reducing efficiency and increasingbelt wear.

SUMMARY OF THE INVENTION

The present invention allows the use of a soft spring while stillmaintaining a high gripping force at low speed during braking.

The driven pulley according to the invention is used in a variable speedtransmission and comprises:

a shaft having two ends;

two coaxial flanges located on the shaft, each flange having an innerconical wall, the inner walls facing each other and providing a V-shapedgroove for a trapezoidal pulley belt exerting substantially a radialforce and a tangential force on the inner walls, one of the flanges,hereinafter called "fixed flange", being rigidly attached at one end ofthe shaft, the other flange, hereinafter called "movable flange", beingslidably and rotatably mounted on the shaft;

a helicoidal spring, mounted around the shaft, forcing the movableflange to get closer to the fixed flange; and

means for generating an angular displacement of the movable flange withreference to the fixed flange and in function of the distancein-between, the angular displacement being in a direction opposite tothe direction of rotation of the pulley.

The object of the present invention is to provide an improvement whichconsists of:

a substantially conical ring rotatably attached to the movable flange,on a side opposite its inner conical wall, and concentric therewith, theconical ring having an inner edge and an outer edge where the inner edgeis farther from the fixed flange than the outer edge;

at least two flyweights, symmetrically disposed around a plate rigidlyattached to the shaft, each flyweight being operatively attached to anarm, the arm being operatively attached to the plate and extendingtherefrom, each of the arm being able to revolve, about a tangentialaxis disposed at an edge of the plate, between a first position wherethe corresponding flyweight projects substantially toward and againstthe annular conical surface of the movable flange and a second positionwhere the corresponding flyweight projects substantially radially withreference to the shaft; and

biasing means for forcing the flyweights to rest against the conicalring, the biasing means generating an axial resultant force for movingthe movable flange closer to the fixed flange, the force being maximumat the first position.

In use, at minimum rotation speed, the flyweights are at the firstposition. As rotation speed increases, the flyweights are subjected to acentrifugal force counterbalancing the biasing means, therefore reducingit and moving the flyweights closer to the second position.

According to a preferred embodiment, the flyweights are not in contactwith the conical ring when at the second position.

According to another preferred embodiment, the biasing means comprise apair of torsion springs coaxially mounted around the tangential axis.

According to a still preferred embodiment, the driven pulley furthercomprises means to have a minimum spacing between the fixed and movableflanges.

A non restrictive description of a preferred embodiment will now begiven with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of the driven pulley, according to theinvention, with the upper half in a low rotation speed position and thelower half in a high rotation speed position.

FIG. 2 is a rear elevation view of the cam plate.

FIG. 3 is a perspective view of the cam plate with only one flyweightpartially mounted thereon.

FIG. 4 is a perspective view of the rear of the movable flange.

FIG. 5 is a cross-sectional view of the driven pulley according to analternative design of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

With reference to the accompanying drawings, the driven pulley comprisethe following numbers:

10: driven pulley

12: fixed flange

14: inner conical wall (fixed flange)

16: movable flange

18: inner conical wall (movable flange)

20: V-shaped groove

30: trapezoidal belt

40: shaft

42: cylinder

44: cylindrical brass

50: helicoidal spring

60: cam plate

62: cam surfaces

64: slider buttons

70: conical ring

72: inner edge

74: outer edge

76: rolls

78: retaining screws

80: flyweight assembly

82: cylindrical flyweight

84: H-shaped arm

86: shaft

88: torsion springs

90: contact surface

100: stopper

With reference to the annexed drawings, the invention relates to animprovement in a driven pulley 10 of a variable speed transmission. Thispulley 10 comprises:

a shaft 40 having two ends;

two coaxial flanges 12 and 16, both located on the shaft 40, each flangehaving an inner conical wall, the inner walls facing each other andproviding a V-shaped groove for a trapezoidal pulley belt exertingsubstantially a radial force and a tangential force on the inner walls,one of the flange, hereinafter called "fixed flange", being rigidlyattached at one end of the shaft, the other flange, hereinafter called"movable flange", being slidably and rotatably mounted on the shaft;

a helicoidal spring 50, mounted around the shaft 40, forcing the movableflange to get closer to the fixed flange and preferably exerting anaxial and a torsional return forces; and

means for generating an angular displacement of the movable flange 16with reference to the fixed flange 12 and in function of the distancein-between, the angular displacement being in a direction opposite tothe direction of rotation of the pulley.

The driven pulley 10, as shown in FIG. 1, is one of the parts of avariable speed transmission which substantially comprises a drivingpulley (not shown), a trapezoidal belt 30 and the driven pulley 10. Thedriven pulley 10 comprises a fixed flange 12 having an inner conicalwall 14, and a movable flange 16 having an inner conical wall 18. Thefixed flange 12 and the movable flange 16 are coaxially mounted. Theinner conical walls 14 and 18 are facing each other and define aV-shaped groove 20. A trapezoidal belt 30 is winded around substantiallythe half of the driven pulley 10, against the inner conical walls 14 and18. Another portion of the trapezoidal belt 30 is winded around thedriving pulley (not shown).

The fixed flange 12 is rigidly attached at one the ends of the shaft 40.The movable flange 16 is slidably and rotatably mounted around the shaft40 by means of a cylinder 42, located at the back of the movable flange16 and rigidly attached thereon. The shaft 40 may be mechanicallyconnected to ground traction means (not shown), such as wheels, tracksor any other suitable mechanism. Advantageously, a cylindrical brass 44,located between the shaft 40 and the cylinder 42, may be provided forpreventing wear and reducing the friction. Means for providing a minimumspacing between the fixed flange 12 and the movable flange 16, such as astopper 100, are provided for preventing the trapezoidal belt 30 fromgetting out of the driven pulley 10. A ring (not shown) of a suitablethickness may also be provided.

This improvement consists of:

a substantially conical ring 70 rotatably attached to the movableflange, on a side opposite its inner conical wall, the conical ring 70having an inner edge 72 and an outer edge 74 where the inner edge 72 isfarther from the fixed flange 12 than the outer edge 74;

at least two flyweights, preferably cylindrical flyweights 82,preferably three, symmetrically disposed around a plate rigidly attachedto the shaft, each flyweight 82 being operatively attached to an arm,the arm being operatively attached to the plate and extending therefrom,each of the arm being able to revolve, about a tangential axis disposedat an edge of the plate, between a first position where thecorresponding flyweight 82 projects substantially towards and againstthe annular conical surface 70 of the movable flange 16 and a secondposition where the corresponding flyweight 82 projects substantiallyradially with reference to the shaft 40; and

biasing means for forcing the flyweights 82 to rest against the conicalring 70, the biasing means having an axial resultant force for movingthe movable flange 16 closer to the fixed flange 12, the force beingmaximum at the first position.

In use, at minimum rotation speed, the flyweights 82 are at the firstposition. As rotation speed increases, the flyweights 82 are subjectedto a centrifugal force counterbalancing the means forcing the flyweightsto rest against the conical ring 70, therefore reducing it and movingthe flyweights closer to the second position.

Preferably, the conical ring 70 is not having a constant angle. It maybe curved or have two or more cone angles, thereby allowing a mechanicalbehaviour adapted to specific designs.

According to particularly preferred embodiments of the invention, themeans to generate angular displacement of the movable flange 16 withrespect to the fixed flange 12 may comprise a cam plate 60 rigidlyattached on the shaft 40 at the end opposite of the fixed flange 12. Ahelicoidal spring 50 is coaxially mounted around the shaft 40 betweenthe movable flange 16 and the cam plate 60.

As shown in FIG. 3, the cam plate 60 may have three cam surfaces 62symmetrically disposed around a circular path. The cam surfaces 62 arecurved to follow the circular path and progressively project toward themovable flange 16 in the direction of rotation of the driven pulley 10.It is not essential to have three cam surfaces 62, but two is theminimum number. Additionally, the cam surfaces 62 may have anon-constant ramp angle according to specific design requirements.

As shown in FIG. 4, three slider buttons 64 are rigidly attached to theback of the movable flange 16, projecting therefrom towards the camplate 60. The slider buttons 64 are symmetrically disposed around acircular path substantially similar to the circular path of the camsurfaces 62. The number of slider buttons 64 is equal to the number ofcam surfaces 62.

It is possible to have the cam surfaces 62 at the back of the movableflange 16 and the slider buttons 64 on the cam plate 60. It is alsopossible to have two sets of cam surfaces 62, one at the back of themovable flange 16 and the other on the cam plate 62, with conicalrollers set in-between (not shown).

As aforesaid, the driven pulley 10 changes its winding diameter of thetrapezoidal belt 30 to keep an optimum ratio between the driving pulleyand the driven pulley, thus compensate for the variation of the windingdiameter of the driving pulley. The power is transmitted from thedriving pulley to the driven pulley 10 by a tangential force generatedby the traction on the trapezoidal belt 30. Half of the power istransmitted to the inner conical wall 14 and then to the shaft 40 bymeans of the fixed flange 12. The other half of the power goes to shaft40 through the inner conical wall 18, the movable flange 16, the sliderbuttons 64, the cam surfaces 62 and the cam plate 60.

As the winding diameter of the driving pulley increases, the tension inthe trapezoidal belt 30 increases. This tension exerts a radial force,oriented towards the center of the driven pulley 10, on the innerconical walls 14 and 18. It thus force the movable flange 16 to moveaway from the fixed flange 12. However, the axial force exerted on themovable flange 16 is counterbalanced by the helicoidal spring 50 and anaxial component of the tangential force exerted by the trapezoidal belt30 on the movable flange 16. If the tension in the trapezoidal belt 30increases, the movable flange 16 is moved away from the fixed flange 12and the spring 50 is depressed until a new equilibrium between thespring force and the axial force generated by the trapezoidal belt 30.Each time the movable flange 16 moves, the winding diameter changes.

Half of the torque from the trapezoidal belt 30 goes through the innerconical wall 18, the movable flange 16 and the slider buttons 64, whichare sliding on the cam surfaces 62, themselves reacting axially on theslider buttons 64, thereby generating an additional gripping force. Whenthe torque is increased, it increases the belt grip and forces thetrapezoidal belt to get to a greater winding diameter.

Additionally, the helicoidal spring 50 may exert a torsional returnforce in addition to the axial return force. In such case, the torsionalreturn force is exerted in the direction of rotation and contributes toreset the flanges 12 and 16 closer to each other.

As also aforesaid, the spring 50 has a mechanical behaviour opposite towhat it should be in an ideal driven pulley, which is to allow a maximumgripping force on the trapezoidal belt 30 when the winding diameter ismaximum and a minimum gripping force when the winding diameter isminimum. This is due to the fact that the more a spring is depressed,the greater the return force becomes.

To allow a maximum gripping force at low speed, such as when the windingdiameter is maximum, an auxiliary device is provided to help the spring50. As shown in FIG. 2, it comprises a set of three flyweight assemblies80 symmetrically disposed around the cam plate 60, at the outer edgethereof. It is not essential to have three flyweight assemblies 80 butthe minimum number is two for balancing purposes.

Each flyweight assembly 80 has a cylindrical flyweight 82 operativelyattached to the cam plate 60 by a H-shaped arm 84, itself operativelyattached to the cam plate 60, and extending therefrom, by means of ashaft 86. Two torsion springs 88, coaxially mounted around the shaft 86and each located at one side of the arm 84, force the flyweight 82 torest against a conical ring 70 provided at the rear of the movableflange 16, near the outer edge 74 thereof, and facing the cam plate 60.One or more than two torsion springs 88 may also be used.

Referring to FIG. 1, the conical ring 70 is rotatably attached to themovable flange 16 for allowing free relative movement between themovable flange 16 and the flyweights 82 when the movable flange 16 isfollowing the cam surface 62. It thus allows a low friction movementwhich do not interfere with the normal operation of the pulley. Theconical ring 70 is rolling on cylindrical rolls 76 and is maintained isplace by screws 78 for preventing it from moving axially. Although balls(not shown) can be used instead of rolls, rolls are preferred becausethe forces is more evenly distributed.

As shown in FIG. 4, the inner edge 72 is closer to the cam plate 60 thanthe outer edge 74. To minimize the friction and to have a bettercontact, each flyweight 82 has a contact surface 90 having a slightlylarger diameter and centrally located thereon, as shown in FIGS. 2 and3.

As shown in FIG. 1, each arm 84 is able to revolve, about a tangentialaxis disposed at an edge of the cam plate 60, between a first positionwhere the corresponding flyweight 82 projects substantially towards andagainst the conical ring 70 of the movable flange 16, and a secondposition where the corresponding flyweight 82 projects substantiallyradially with reference to the shaft 40. Each flyweight 82 can turnfreely on itself around its longitudinal axis.

In use, at low speeds, the flyweights 82 apply a force on the conicalring 70 and force the movable flange 16 to get closer to the fixedflange 12. At high speeds, the flyweights 82 are subjected to acentrifugal force counterbalancing the return force of the torsionsprings 88 and therefore the flyweights 82 are brought closer to thesecond position as rotation speed increases.

At the second position, the flyweights 82 are not in contact with theconical ring 70. However, this is not essential.

Initially, as shown in FIG. 1 at the upper half of the drawing, theH-shaped arm 84 is almost parallel to the shaft 40. The radial forceexerted by the trapezoidal belt 30 will then have to be very importantto move the flyweights 82 from the first position. But as the rotationspeed increases, so does the centrifugal force acting on the flyweights82 and the movable flange 16 will then be easier to move by thetrapezoidal belt 30 despite the return force generated by the torsionsprings 88, which exert a force on the arm 84 to get the flyweights 82to the first position. Yet, as the rotation speed increases, the actionof the flyweight 82 is less important, thus exactly as it should be in aideal driven pulley.

At a certain rotation speed, the flyweights 82 leave the conical ring70. They reach the second position just after they took off and theyremain there until the rotation speed is lower.

With the use of the flyweight assemblies 80, it is now possible to usesoft spring and still have a suitable gripping force at low speed. Theuse of the soft spring 50 also allows to have a reasonably low grippingforce at high speeds, therefore better efficiency.

Of course, as shown in FIG. 5, the position of the conical ring 70 andthe flyweight assemblies 80 may be inverted.

Although a preferred embodiment of the invention has been described indetail herein and illustrated in the accompanying drawings, it is to beunderstood that the invention is not limited to this precise embodimentand that various changes and modifications may be effected thereinwithout departing from the scope or spirit of the invention.

What I claim is:
 1. In a driven pulley of a variable speed transmission, the pulley comprising:a shaft having two ends; two coaxial flanges located on the shaft, each flange having an inner conical wall, the inner walls facing each other and defining a V-shaped groove for a trapezoidal pulley belt exerting substantially a radial force and a tangential force on the inner walls, one of the flanges, hereinafter called "fixed flange", being rigidly attached at one end of the shaft, the other flange, hereinafter called "movable flange", being slidably and rotatably mounted on the shaft; a helicoidal spring, mounted around the shaft, forcing the movable flange to get closer to the fixed flange; and means for generating an angular displacement of the movable flange with reference to the fixed flange and in function of the distance in-between, the angular displacement being in a direction opposite to the direction of rotation of the pulley; the improvement consisting of: a substantially conical ring rotatably attached to the movable flange, on a side opposite its inner conical wall, and concentric therewith, the conical ring having an inner edge and an outer edge where the inner edge is farther from the fixed flange than the outer edge; at least two flyweights, symmetrically disposed around a plate rigidly attached to the shaft, each flyweight being operatively attached to an arm, the arm being operatively attached to the plate and extending therefrom, each of the arm being able to revolve, about a tangential axis disposed at an edge of the plate, between a first position where the corresponding flyweight projects substantially toward and against the annular conical surface of the movable flange and a second position where the corresponding flyweight projects substantially radially with reference to the shaft; and biasing means for forcing the flyweights to rest against the conical ring, the biasing means generating an axial resultant force for moving the movable flange closer to the fixed flange, the force being maximum at the first position; whereby, at minimum rotation speed, the flyweights are at the first position, and, as rotation speed increases, the flyweights are subjected to a centrifugal force counterbalancing the biasing means, therefore reducing it and moving the flyweights closer to the second position.
 2. In a driven pulley of a variable speed transmission, the pulley comprising:a shaft having two ends; two coaxial flanges located on the shaft, each flange having an inner conical wall, the inner walls facing each other and defining a V-shaped groove for a trapezoidal pulley belt exerting substantially a radial force and a tangential force on the inner walls, one of the flanges, hereinafter called "fixed flange", being rigidly attached at one end of the shaft, the other flange, hereinafter called "movable flange", being slidably and rotatably mounted on the shaft; a helicoidal spring, mounted around the shaft, for forcing the movable flange to get closer to the fixed flange; a cam plate rigidly attached to one end of the shaft opposed to the end where the fixed flange is attached, the cam plate having at least two cam surfaces symmetrically disposed around a circular path, the cam surfaces having an end projecting towards the movable flange; and slider buttons in number equal to the cam surfaces, rigidly attached to the movable flange and projecting therefrom towards the cam plate, symmetrically disposed around a circular path substantially similar to the circular path of the cam surfaces, the slider buttons being made of a low friction material and having a shape suitable for easily sliding on the cams; the improvement comprising: a substantially conical ring rotatably attached to the movable flange, on a side opposite its inner conical wall, and concentric therewith, the conical ring having an inner edge and an outer edge where the inner edge is closer to the cam plate than the outer edge; at least two cylindrical flyweights symmetrically disposed around the cam plate, each flyweight being operatively attached to an arm, the arm being operatively attached to the cam plate and extending therefrom, each of the arm being able to revolve, about a tangential axis disposed at an edge of the cam plate, between a first position where the corresponding flyweight projects substantially towards and against the annular conical surface and a second position where the corresponding flyweight projects substantially radially with reference to the shaft; and biasing means for forcing the flyweights to apply a force on the conical ring having an axial resultant force for moving the movable flange closer to the fixed flange, the force being maximum at the first position; whereby, at minimum rotation speed, the flyweights are at the first position, and, as rotation speed increases, the flyweights are subjected to a centrifugal force counterbalancing the biasing means, therefore moving the flyweights closer to the second position.
 3. An improved driven pulley according to claim 2, wherein the flyweights are not in contact with the conical ring when at the second position.
 4. An improved driven pulley according to claim 3, wherein their is three cam surfaces.
 5. An improved driven pulley according to claim 4, wherein the number of cylindrical rollers is three.
 6. An improved driven pulley according to claim 5, wherein the biasing means comprise a pair of torsion springs coaxially mounted around the tangential axis.
 7. An improved driven pulley according to claim 2, wherein the helicoidal spring exerts an axial and a torsional return forces.
 8. An improved driven pulley according to claim 2, further comprising means for providing a minimum spacing between the fixed and movable flanges.
 9. An improved driven pulley according to claim 1, wherein the substantially conical ring is not flat.
 10. An improved driven pulley according to claim 2, wherein the substantially conical ring is not flat.
 11. In a driven pulley of a variable speed transmission, the pulley comprising:a shaft having two ends; two coaxial flanges located on the shaft, each flange having an inner conical wall, the inner walls facing each other and defining a V-shaped groove for a trapezoidal pulley belt exerting substantially a radial force and a tangential force on the inner walls, one of the flanges, hereinafter called "fixed flange", being rigidly attached at one end of the shaft, the other flange, hereinafter called "movable flange", being slidably mounted around the shaft and able to slide and turn thereon; a helicoidal spring, mounted around the shaft, for forcing the movable flange to get closer to the fixed flange, the helicoidal spring exerting an axial and a torsional return forces; a cam plate rigidly attached to one end of the shaft opposed to the end where the fixed flange is attached, the cam plate having three cam surfaces symmetrically disposed around a circular path, the cams having an end projecting towards the movable flange; three slider buttons, rigidly attached to the movable flange and projecting therefrom towards the cam plate, symmetrically disposed around a circular path substantially similar to the circular path of the cams, the slider buttons being made of a low friction material and having a shape suitable for easily sliding on the cams; and means for providing a minimum spacing between the fixed and movable flanges; the improvement comprising: a substantially conical ring rotatably attached to the movable flange, on a side opposite its inner conical wall, and concentric therewith, the conical ring having an inner edge and an outer edge where the inner edge is closer to the cam plate than the outer edge, the substantially conical ring being not flat; three cylindrical flyweights symmetrically disposed around the cam plate, each flyweight being operatively attached to an arm, the arm being operatively attached to the cam plate and extending therefrom, each of the arm being able to revolve, about a tangential axis disposed at an edge of the cam plate, between a first position where the corresponding flyweight projects substantially toward and against the annular conical surface of the movable flange and a second position where the corresponding flyweight projects substantially radially with reference to the shaft; and biasing means for forcing the flyweights towards the first position, the biasing means comprising a pair of torsion springs coaxially mounted around the tangential axis; whereby, when the driven pulley is in rotation, at low rotation speed, the flyweights apply a force on the conical ring, contributing to force the movable flange to get closer to the fixed flange, and, at higher rotation speed, the flyweights are subjected to a centrifugal force counterbalancing the pair of torsion springs coaxially mounted around the tangential axis, and therefore the flyweights are brought closer to the second position as rotation speed increases, the flyweights being not in contact with the conical ring when at the second position.
 12. In a driven pulley of a variable speed transmission, the pulley comprising:a shaft having two ends; two coaxial flanges located on the shaft, each flange having an inner conical wall, the inner walls facing each other and defining a V-shaped groove for a trapezoidal pulley belt exerting substantially a radial force and a tangential force on the inner walls, one of the flanges, hereinafter called "fixed flange", being rigidly attached at one end of the shaft, the other flange, hereinafter called "movable flange", being slidably and rotatably mounted on the shaft; a helicoidal spring, mounted around the shaft, forcing the movable flange to get closer to the fixed flange; and means for generating an angular displacement of the movable flange with reference to the fixed flange and in function of the distance in-between, the angular displacement being in a direction opposite to the direction of rotation of the pulley; the improvement consisting of: a substantially conical ring rotatably attached to the movable flange, on a side opposite its inner conical wall, and concentric therewith, the conical ring having an inner edge and an outer edge, the inner edge being farther from the fixed flange than the outer edge; at least two flyweights, symmetrically disposed around the movable flange on a side opposite to the inner conical wall thereon, each flyweight being operatively attached to an arm, the arm being operatively attached to the movable flange and extending therefrom, each of the arm being able to revolve, about a tangential axis disposed at an edge of the plate, between a first position where the corresponding flyweight projects substantially toward and against the annular conical surface and a second position where the corresponding flyweight projects substantially radially with reference to the shaft; and biasing means for forcing the flyweights to rest against the conical ring, the biasing means generating an axial resultant force for moving the movable flange closer to the fixed flange, the force being maximum at the first position; whereby, at minimum rotation speed, the flyweights are at the first position, and, as rotation speed increases, the flyweights are subjected to a centrifugal force counterbalancing the biasing means, therefore reducing it and moving the flyweights closer to the second position. 